Ion mobility spectrometry (IMS) is a rugged, inexpensive, sensitive, field portable technique for the detection of organic compounds. It is widely employed in ports of entry and by the military as a particle detector for explosives and drugs of abuse. Many organic high explosives do not have a high enough vapor pressure for effective vapor sampling. However, these explosives and their commercial explosive mixtures have characteristic volatile components detectable in a headspace when contained. In addition, taggants are added to commercially-manufactured explosives to aid in detection through headspace sampling. Solid phase microextraction (SPME) is an effective extraction technique that has been successfully employed in the field for the absorption and/or adsorption of a variety of compounds. SPME can easily extract these compounds from the headspace for IMS vapor detection.
In 1996, Congress passed the Anti Terrorism Bill, which requires the addition of detection taggants to plastic explosive compounds and the ban of sale or import of plastic explosives that do not contain a detection agent [Public Law 104-132, Antiterrorism and Effective Death Penalty Act of 1996; Section 603; Apr. 24, 1996]. A detection taggant is a solid or liquid vapor emitting substance added to an explosive material to facilitate discovery before detonation. A detection taggant can easily be detected by explosive vapor detectors, such as ion mobility spectrometers (IMS). IMS are currently used as particle detectors requiring the manual transfer of particles from a suspect area and thermal desorption into the spectrometer.
Many organic high explosives, particularly those found in plastic explosives, do not produce a significant vapor pressure to allow headspace vapor detection, especially in the field [R. G. Ewing, C. J. Miller, Field Anal. Chem. Technol., 5:215-221 (2001)]. The International Civil Aviation Organization (ICAO) has designated the detection taggant compounds as the following four compounds in the indicated concentrations by mass: 0.5% 2-nitrotoluene (2-NT), 0.5% 4-nitrotoluene (4-NT), 0.1% 2,3-dimethyl-2,3-dinitrobutane (DMNB), and 0.2% ethylene glycol dinitro (EGDN) [Convention on the Marking of Plastic Explosives for the Purpose of Detection, http://www.mcgill.ca/files/ias1/montreal1991.pdf, accessed Feb. 6, 2008]. These compounds were selected because they are not commonly found in nature, they do not hinder the explosive properties of the tagged compound, they continue to release their vapors at a steady rate for 5 to 10 years, they do not present a significant environmental hazard, and they do not readily adhere to common substances the taggant may come in contact with [J. Yinon, Forensic Applications of Mass Spectrometry, CRC Press, Boca Raton, Fla., 1995]. In addition to taggant detection to determine if an improvised explosive device is present, there are also additional odor signature compounds present in significant amounts for detection [M. Williams, J. M. Johnston, M. Cicoria, E. Paletz, L. P. Waggoner, C. Edge, S. F. Hallowell, Proc. SPIE, 35:291-301 (1998)]. These odor signature compounds can be extracted using SPME and detected by IMS [J. M. Perr, K. G. Furton, J. R. Almirall, J. Sep. Sci., 28:177-183 (2005). 2. H. Lai, P. Guerra, M. Joshi, J. R. Almirall, J. Sep. Sci., 31: 402-412 (2008)].
The Transportation Security Administration (TSA) has mandated that all airports in the United States screen all bags for explosives [http://www.aviationnow.com/avnow/news/channel_comm.jsp?view=story & id=news/captl130.xml, accessed Mar. 12, 2003]. IMS is one of the screening tools approved by the TSA. These instruments can detect the taggants selected by the ICAO [R. G. Ewing, D. A. Atkinson, G. A. Eiceman, G. J. Ewing, Talanta, 54:515-529 (2001)]. Detector dog teams are another widely used screening tool [D. S. Moore, Rev. Sci. Instrum., 75:2499-2512 (2004)].
IMS is a presumptive detection method for organic compounds that is extremely fast, straightforward to use, low cost, with clear-cut data interpretation, excellent sensitivity, and low power demands. Run times for commercial IMS range from 1 to 7 seconds. IMS machines have a large installed base of over 10,000 commercial instruments and 50,000 military instruments conducting over 10,000,000 analyses per year [K. Cottingham, Anal. Chem., 75:435A-439A (2003)]. The false positive rate for swabbing of suspected areas is reported to be less than 1% while the false positive rate for air sampling of suspected areas is less than 0.1% [Itemiser Contraband (Drug and Explosive) Detection and Identification System User's Manual revision 3.1., GE Ion Track Instruments, Wilmington, Mass. (1999)]. For example, when seventeen of the most likely false positives for 2,4,6-trinitrotoluene (2,4,6-TNT) were studied, it was found that only seven of those were detected by the IMS and upon careful analysis, the compound that displayed the most similar mobility to TNT, 4,6-dinitro-o-cresol (4,6-DN-o-C), did not produce a false positive [L. M. Matz, P. S. Tornatore, H. H. Hill, Talanta, 54:171-179 (2001)]. IMS has also been evaluated as a field screening application and found to have a number of advantages over other field deployable techniques [H. H. Hill, G. Simpson, Field Anal. Chem. Technol., 1:119-134 (1997)].
In IMS, ions are separated and recognized on the basis of their mobility values. Some instruments can analyze only positive or negative ions in a determination, while other instruments can analyze both positive and negative ions in the same analytical determination. The detection of explosives and taggants are typically conducted in the negative ion mode. Mobility (K in cm2/V s) is determined using the drift velocity (vd in cm/s) of the ion through a heated drift tube and a weak electric field (E in V/cm) that the ion is exposed to when inside the heated drift tube (vd=E×K). Ionization occurs in the reaction region when a 63Ni source initiates ionization by emitting β particles. The β particles trigger a cascade of ionization reactions, either with the air or with a dopant gas present in the ionization region, to produce reactant ions. The reactant ions interact with the sample through ion molecule interactions to generate product ions that are detected during the analysis. Other ionization methods can be used, such as a tritium β particle emitter [J. W. Leonhardt, J. Radioanal. Nucl. Chem., 206:333 (1996)], photoionization [D. D. Lubman, M. N. Kronick, Anal. Chem., 54:1546 (1982); C. S. Lesure, M. E. Fleischer, G. K. Anderson, G. A. Eiceman, Anal. Chem., 58:2142 (1996); H. Borsdorf, H. Schelhorn, J. Flachowsky, H. Döring, J. Stach, Anal. Chim. Acta., 403:235-242 (2000)], and corona discharge [R. A. Miller, E. G. Nazarov, G. A. Eiceman, T. A. King, Sens. Actuators A, 91:301-312 (2001)], but ionization using the 63Ni source is the most common. Ionization results in either molecular ion or molecular clusters related to the molecular ion. Fragmentation is a rare occurrence but it has been observed in very special cases [K. Cottingham, Anal. Chem., 75:452A (2003)].
Also during IMS, an electronic gate opens at timed intervals throughout the run to allow the ions to enter the drift region for separation to occur. The opening of the electronic gate begins the timing of the ion's flight time to reach the detector in order to calculate the drift velocity. A linear potential drop exists in the drift region to move the reactant and product ions towards the detector. Neutrals and ions of the opposite charge being analyzed are swept out of the drift region by a counter-current flow of drift gas. A plasmagram results as a plot of the current measured at the collector electrode with respect to time in the millisecond (ms) time frame. The General Electric Ion Track Itemiser® 2 collects one plasmagram every 100 ms. For a 7 s run, 70 plasmagrams are recorded. The 70 collected plasmagrams then undergo a data deconvolution step in which a representative plasmagram is produced. An intensity map views all the plasmagrams collected during one run stacked on each other showing height as intensity. Dark areas represent peaks while lighter areas represent troughs. A single plasmagram can be imported into Excel and graphed.
An important factor that affects mobility is the collisional cross-sectional area (Ωd in Å2). The mean free path of an ion with a large collisional cross-section is shorter than those of a smaller collisional cross-section. If two molecules have the same collisional cross-section, the heavier molecule will have a longer mean free path due to its slower velocity. The dopant gas and air within the drift tube affect the drift velocity (vd) by collisions, making IMS a quasi mass analyzer but instead of using only mass to charge (m/z) it uses three different parameters: shape (collisional cross-section), mass, and charge.
SPME is a highly effective sample extraction technique that has been shown to be an effective tool for the analysis of volatile and semi-volatile components and was named one of the six great ideas in analytical chemistry of the last decade [K. G. Furton, J. Wang, Y. L. Hsu, J. Walton, J. R. Almirall, J. Chromatogr. Sci., 38:297-306 (2000); K. G. Furton, J. R. Almirall, M. Bi, J. Wang, W. Lu, J. Chromatogr. A, 885:419-432 (2000); K. P. Kirkbride, G. Klass, P. E. Pigou, J. Forensic Sci., 43:76-81 (1998); J. Handley, C. M. Harris, Anal. Chem., 73:23A-26A (2001)]. Volatile or semi-volatile compounds are extracted by absorption and/or adsorption onto a non-volatile polymeric coating or solid sorbent phase. After the analytes are sorbed onto the SPME phase they are commonly volatilized by thermal desportion in an injection port. SPME devices come in a variety of forms including but not limited to: articles coated in the SPME phase, vessels lined with the SPME phase, and SPME coated stir bars. A common and commercially available form of SPME is a fiber configuration. SPME has been successfully applied to the recovery of explosives and explosive vapors followed by GC/MS and HPLC analysis [J. R. Almirall, L. Wu, M. Bi, M. W. Shannon, K. G. Furton, Proc. SPIE, 35:18-23 (1999); K. G. Furton, L. Wu, J. R. Almirall, J. Forensic Sci., 45:845-852 (2000); K. G. Furton, R. J. Harper, J. M. Perr, J. R. Almirall, Proc. SPIE, 5071:183-192 (2003)]. Polydimethyl siloxane (PDMS) fibers have been reported as the most effective and rugged fiber for rapid headspace extraction of explosives with the least amount of carry-over problems [N. Lorenzo, T. Wan, R. J. Harper, Y. Hsu, M. Chow, S. Rose, K. G. Furton, J. Anal. Bioanal. Chem., 376:1212-1224 (2003)] for explosive compounds.
SPME is a very effective tool for the extraction of taggants from headspace samples under ambient environmental conditions that can also be used for remote sampling. Ion mobility spectrometry is a very effective tool for detecting trace amounts of explosives and explosive taggants under ambient environmental conditions [M. Nambayah, T. I. Quickenden, Talanta, 63:461-467 (2004)]. Ion mobility spectrometers have been successfully interfaced to other sample introduction techniques such as a solid phase extraction (SPE) [T. L. Buxton, P. B. Harrington, Appl. Spectrosc., 57:223-232 (2003)], gas chromatography (GC) [J. P. Dworzanski, W. H. McClennen, P. A. Cole, S. N. Thornton, H. L. C. Meuzelaar, N. S. Arnold, A. P. Snyder, Field Anal. Chem. Technol., 1:295-305 (1997)], and liquid chromatography (LC) [S. J. Valentine, M. Kulchania, C. A. S. Barnes, D. E. Clemmer, Int. J. Mass Spectrom., 212:97-109 (2001)].
A SPME-IMS interface has been created to couple the extraction efficiency of SPME to the detection capability of IMS. The demand for this sort of field portable, remote, reliable sampling is high [J. Yinon, Anal. Chem., 75:99A-105A (2003)]. The SPME-ISM interface shown meets this need by extracting vapors from explosives, taggants in explosives, controlled substances, biohazards, and mixtures thereof (detectable target vapors or detectable vapors) from a headspace for subsequent detection by a commercially available IMS in a simple, rapid, sensitive, and inexpensive manner.
Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximates, by use of the antecedent “about” it will be under that the particular value forms another embodiment.